How big a capacitor can store energy in a battery

Li-ion batteries can achieve energy densities up to 650 watt-hours per liter (Wh/L), while even the most advanced supercapacitors offer only around 10 Wh/L or 1. A capacitor imposes an electric field around a dielectric, which can only store energy until it breaks down (typically a runaway ionization process). Ionization requires a few eV/atom to occur, but it can be triggered at much lower field strengths per atom/molecule, because a free charge moving. While a battery stores energy in chemical form, converting it back into electrical energy as needed, a capacitor stores energy in an electric field. Capacitance depends on the surface area of the conductive plates, the distance between the plates, and the properties of the dielectric material. This simple capability enables capacitors to smooth voltage ripples, filter signals, couple AC while blocking DC, provide temporary power during brief interruptions, set timing intervals.